In order to implement an optical transceiver made of one chip that covers the capacity of a class of terabits per second, it is necessary to use single mode optical fibers as the optical fibers used for light transmission in the case of wavelength division multiplexing or long distance transmission.
Si photonic wire waveguides used for silicon PIC's have a size as small as one μm or less, and therefore are appropriate for high integration. However, the size is greatly different from that of single mode optical fibers, and therefore, it is a theme to enhance the efficiency in the coupling of the Siphotonic wire waveguides with optical fibers.
According to a conventional method, a spot size converter of which an end is tapered is used to make the spot size compatible with an optical fiber in order to enhance the efficiency in the coupling with an optical fiber. FIGS. 15A and 15B are diagrams illustrating a conventional optical fiber mounted photonic integrated circuit device. FIG. 15A is a cross-sectional diagram along the optical axis, and FIG. 15B is a plan diagram where the main portions can be clearly seen from the top.
An SOI wafer is used in such a manner that a Si fine wire core 63 having a thickness of 0.25 μm is provided on top of a silicon substrate 61 with a BOX layer 62 that also works as a lower clad layer in between. A spot size converter 64 of which an end is tapered is provided to an end of the Si fine wire core 63. A SiO2 film is provided on the upper surface of the Si fine wire core 63 as an upper clad layer 65, and a wide SiON core 66 is provided so as to cover the sides of the spot size converter 64. A single mode optical fiber 67 is arranged so that the core 68 faces the SiON core 66. Here, 69 and 70 in the figures are a clad and a ferrule, respectively.
As illustrated in the figures, the propagating light beam having a spot diameter of approximately 0.25 μm is expanded through the spot size converter 64 so that the spot diameter is approximately 9 μm in the SiON core 66 when the light beam enters into the core 68 of the single mode optical fiber 67.